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Snubber Circuit Design Calculators

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Driving inductive loads with transistor switches, whether they be flyback transformers, relays or motors often result in the high voltage resonant spikes when the coils are interrupted from their current current source by the transistor.  There are various ways of mitigating these undesirable spikes which cause component failures and EMI issues.  The most common approach is to use snubber circuits.   This engineering tutorial article not only explains common snubber circuits but provides several design calculators, that make snubber design easy.  

Figure 1 - Coil driving circuit showing the use of a RCD clamping snubber (D1,C1,R1) and a RCD rate of voltage rise snubber (D2,R2,C2).

Method 1 - RCD Rate of Voltage Rise Snubber Circuit

With this method we want to limit the rise of the voltage when the transistor switch is shut off. 

Another way of stating the problem is that we want the drain or collector of the transistor to snap back to the rail voltage in dt seconds. 

When the transistor shuts off the current is at it's peak value.  We can use the following equation to find the necessary capacitance to limit the rise time to a given value. 

Ipk= C*dV/dt

C= Ipk*dt/Vrail

We want to only affect the leading edge of the rising voltage and so the time constant (TC) of the RC element must be a fraction of the period.   We use the time constant formula for choosing R. 

RC= TC

A rule of thumb is to choose Tc to be 1/10th the switch on time. 

R= TC/(C*10)= 1/(f*C*2*10)= 1/(f*C*20)

Energy stored in the capacitor is fully dissipated on each cycle through the resistor, so the power that the resistor must dissipate is: 

P= E_c*f= (1/2*C*V²)*f

When determining the values of the rise time snubber components empirically there are several relationships that can be exploited.  

• With all components fixed and lowering the resistance value, the rise time will decrease and the resistor will consume less power. 
• With all components fixed and raising the capacitance, the rise time will increase and the resistor will consume more power. 
• Changing  the rise time will affect an adjacent clamping snubber.  If the rise time increases it will take energy away from the clamping snubber, and likewise if the rise time decreases it will add to the energy that the clamping snubber must remove from the circuit. 

Here are some other tips: 

• Make sure that the diode can handle the peak current.  
• Reduce the capacitor if the resistor is getting too hot.
• Make sure all of the components can handle the peak voltages. 

Method 2 - RCD Clamping Snubber Circuit

The clamping snubber is essentially a peak detector circuit used for AM demodulation consisting parallel cap and resistor which is fed by a rectifier.  The RC filter is designed to tract the envelope of the AM wave.  In this case where the switching frequency is normally constant, the RC circuit will have a ripple induced by the peak inductor current. The voltage can be clamped to a desired maximum dV value by selecting an appropriate valued capacitor.   The purpose of the resistor is to fully  dissipate the charge on the capacitor each cycle.  

This circuit is often found in power supply circuits and they are used to clip the ringing that occurs because of the resonance of the leakage inductance with the output capacitance of the transistor. MOSFETs often have a maximum drain to source voltage which should not be exceeded.  The clamp can be designed so that this maximum voltage is never exceeded.  The value of the capacitor can be determined by selecting how much voltage change can occur. This delta V is denoted as dV, and can be considered a ripple voltage.  We look at the energy flowing out of the coil into the capacitor, and find that the initial energy of the capacitor plus the inductor energy is equal to the fully charged capacitor:

 E_C0 +E_L= E_CT

1/2*C*V²+1/2*L*I²= 1/2*C*(V+dV)²

In this case I is the peak current of the inductor, and V is the lowest voltage seen on the RC circuit.  Solving for C:

C= L*I²/(dv(dv+2V))

R is chosen such that the time constant is long relative to the period; a time constant of 10 times the period of the switching frequency would be a good choice. 

R= 10/(f*C)

The power is equal to the energy from the inductor dissipated into the resistor each second. 

P= F/2*L*I²

Follow these tips when fine tuning the clamping snubber:

• The diode selection is critical, make sure that the diode can handle a voltage greater than any of the spikes in the circuit, and that it can handle the peak current.  It's better to over-specify this one component from the start. 
• Monitor the voltage between the diode and the resistor and cap. This should be a nearly DC value. If it is choppy then either the capacitor is to small, or the diode has too low of a voltage rating. 
• There is a trade off between the clamp voltage and the power that the circuit dissipates.  The lower the snubber voltage, the hotter all of the components will get.  
• Make sure the capacitor has a high voltage rating.  The rating must be greater than the expected clamping voltage.  
• Adjust the clamping voltage by raising or lowering the resistance, with all other components fixed. 

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